Approximately 20% of human immunodeficiency virus (HlV)-positive individuals are also infected with hepatitis C virus (HCV). Recent studies report that HIV accelerates hepatitis C disease course, perhaps due to loss of immune control of HCV. We have identified two virological variables associated with more severe hepatitis C disease: decreased genetic variability of HCV quasispecies (QS), and increased indices of HCV replication within infected livers. Our preliminary studies suggest that HCV immune responses may be related to both variables. We present herein new evidence that HCV productively replicates in T lymphocytes residing in peri-hepatic lymph nodes in vivo during natural infection, suggesting a novel mechanism of disease. The research proposed in this competitive renewal application will extend our focus on the virology and immunology of chronic HCV infection in subjects with and without HIV coinfection. Aim 1 will address tissue compartmentalization of HCV QS variants within liver and PBMCs of HCV-monoinfected and HCV/HIV coinfected subjects. We predict that HIV coinfection will lead to an increase in HCV replication in peripheral blood cells, and that PBMC-restricted HCV QS will become a more predominant constituent of serum-associated HCV QS. Aim 2 will address the following hypotheses: 1) intrahepatic immune responses are associated with down-regulation of HCV replication in vivo;2) host immune responses drive HCV QS diversification, and 3) compartment-specific immunity occurs during chronic HCV infection. Aim 3 proposes to study the infectivity of HCV QS isolated from study subjects in cell culture in vitro. This Aim will test the hypotheses that certain HCV QS inherently replicate better in hematopoetic cells compared to hepatic cell types (and visa versa), and that HIV coinfection facilitates HCV replication in blood cell types. The proposed studies will be the first to simultaneously assess anti-HCV immune responses in liver versus peripheral blood and their effect on intrahepatic HCV replication and HCV QS variability in serum, liver and blood specimens collected from our research subjects on the same day. The study will also examine the effect of HIV coinfection on HCV-host biology in vivo and HCV-HIV interactions in tissue culture in vitro, contributing to our understanding of how coinfection accelerates hepatitis C disease.